1. Field of Invention
Embodiments of the present invention are related to signal processing, and more particularly, to signal processing by a digital broadcast receiver.
2. Related Art
Conventionally, digital wireless communication systems are designed to provide reliable reception under multiple channel impairments such as additive noise and multi-path fading. A typical wireless digital communication system implements methods for protecting the transmitted data. Such methods usually include error protection coding that introduces data redundancy into a data stream. A wireless receiver can use this data redundancy in order to correct errors that are introduced to the data stream over a wireless medium.
In addition to error protection, many systems incorporate time interleaving. In time interleaving, a continuous sequence of data is interleaved over time, such that a consecutive bit sequence before the interleaving operation becomes non-consecutive. As a result, the bit sequence is transmitted non-consecutively over time. The data sequence is, therefore, transmitted non-sequentially over the air. This interleaving, in combination with error protection coding, can correct bursts of error in the received data. As a result, the bit error rate (BER) after the error correction may be lower when compared to an error rate without the error correction.
In a commercial wireless communication system, the error protection provided by a transmission system is designed to ensure that, under the most extreme reception condition for which the system is designed, the decoded BER is below a certain desired threshold. This BER threshold is referred to as a maximum allowed BER (MAB) threshold. The MAB threshold depends on the type of service that the communication system provides. Below the MAB threshold, quality of the service provided by the system meets a desired quality. For example, in digital audio broadcasting systems, the MAB threshold may be set such that the user is unable to detect any degradation in the received audio quality. This BER threshold is also called threshold of audible impairments (TAI).
A commonly used method for modulating data onto a wireless channel is orthogonal frequency division multiplexing (OFDM), where digital information is modulated on a plurality of carriers that are orthogonal to each other. An OFDM symbol comprises a signal containing a plurality of carriers over a finite interval, called the OFDM symbol interval. A sequence of OFDM symbols comprises a transmitted wireless signal. One exemplary OFDM transmission system is the IBOC system, described in the NRSC-5A standard. Another exemplary OFDM transmission system is DAB/TDMB described in the ETSI EN300401 standard.
Under typical reception conditions, the error protection that a common OFDM transmission system can provide exceeds the protection needed to achieve the MAB threshold (i.e., the decoded BER is lower then the MAB). The difference between the MAB threshold and the decoded BER is referred to as a reception coding margin (RCM). It is not uncommon that some percentage of the received information prior to the error correction can be erased at the receiver and the resulting decoded BER may still be lower or equal to the MAB threshold. The amount of data that can be erased is a function of the RCM; the greater the RCM, the more information that can be erased.
Receivers can consume considerable power in performing the functions of signal reception, signal processing, and error correction sufficient to achieve the desired quality. Therefore, there is a need for power conservation at the receiver.